The present invention relates to optical parametric testing, and more particularly to an emulation device for emulating a polarization mode dispersion (PMD) of an optical transmission system, a compensation device using such an emulation device, a measurement device for measuring a polarization mode dispersion (PMD)-related distortion of an optical signal, a compensation device using such a measurement device and a compensation device for compensating polarization mode dispersion (PMD)-related distortions in an optical transmission system.
Over the last decade, optical waveguides (glass fibers) have been increasingly used for the transmission of digital data streams. When data transmission is to be realized over a long transmission distance, so-called single-mode fibers (SMF) with low absorption coefficients are typically used. As a result of the polarization of electromagnetic radiation, the basic mode of such a single-mode fiber (hereinafter abbreviated as SMF) consists of two modes whose natural waves oscillate orthogonally with respect to one another. As a consequence, two polarization modes propagate in an SMF.
What is problematic about this is the fact that, for manufacturing reasons, the optical fibers are never fully isotropic, but can exhibit a weak, particularly geometrically caused double refraction. Hence, a change of the geometric dimension of an SMF core from an ideal circular to an elliptical cross-section has the consequence that the effective refractive indices along the elliptical semi-axes differ from one another. Mechanical stresses, too, can lead to a direction-dependent refractive index in a circular cross-section.
In this case, the two propagation methods, i.e., polarization modes, in the SMF have different group velocities so that the phase difference between the orthogonal modes does not remain constant. This change in phase difference leads to a change in the polarization state of the optical signal. Moreover, there is a strong coupling between the polarization modes of the SMF so that the influence of the double refraction in the SMF is subject to statistical fluctuations. Mechanical influences, such as minor bending of the SMF or vibrations as well as temperature changes, produce changes in the double refraction.
As a result of the different group velocities of the two mutually orthogonally oscillating natural modes of the SMF, there is what is known as polarization mode dispersion (PMD). The transit time delay between the two partial modes is referred to as differential group delay (DGD). Its mean value is called PMD delay and expressed by a PMD delay coefficient of units ps/km1/2. Consequently, the polarization mode dispersion increases with the square root of the transmission line length. Modern SMFs with minimized PMD delay coefficients are today specified with less than 0.1 to 0.5 ps/km1/2.
The transmission distance realizable under data loss considerations is inversely proportional to the square of the bit rate and to the square of the PMD delay coefficient, which is a parameter of the SMF. While PMD still constitutes a relatively small disturbing influence in optical fiber transmission systems with a transfer rate of 2.5 Gbit/s, it is a serious problem for modern 10 Gbit/s systems, and particularly for future 40 Gbit/s systems, which has a lasting adverse effect on the bit error rate.
Moreover, the PMD of an optical transmission system exhibits a pronounced wavelength dependency (second-order PMD: SOPMD, PMD2). Since in modern optical fiber transmission systems the transmission of a plurality of so-called channels with different optical wavelengths occurs through one SMF, there is for each channel, i.e., for each wavelength, a different PMD that leads to signal distortions.
The distortions in optical transmission systems which are produced by PMD have to be compensated for in high-rate data transfers in order to retain the signal quality and minimize the bit error rate. For this purpose, adaptive PMD compensators have been proposed which are incorporated into an optical transmission system on the receiver side in order to minimize or compensate for the signal-distorting influence of the transmission line PMD. Literature teaches different solution approaches to PMD compensation. In particular, electronic signal conditioners have been proposed which perform a PMD compensation at an electronic level following optoelectronic conversion of the optical transmission signal. It has been shown, however, that the correction of the “optical problem” at the electronic level is only promising to a limited extent.
More promising is a correction of the PMD-distorted signal at the optical level. The point of connection for such a PMD compensator is the DGD and the principal state of polarization (hereinafter abbreviated as PSP) at the output of the transmission system at the medium wavelength λ0 of the optical signal. The task of the PMD compensator (PMDC) is to continuously measure the PMD of the transmission line and to emulate on the basis of the PMD determined a “compensation line” with a PMD transmission characteristic which is such that the optical signal, having traveled through said “compensation line”, i.e., the emulator, is “equalized” or restored. It is advantageous if the measurement of the PMD-related distortions occurs after the signal has traveled through the compensation line. The PMD which still remains uncompensated is thus detected.
The main components of a PMD compensator are thus                a measurement device that continuously measures the PMD of the optical transmission system,        a controllable emulator that can reproduce as accurately as possible a transmission line with an adjustable PMD, and        a control device that is connected to the measurement device and the emulator to adaptively control the emulator as a function of the measurement output signal of the measurement device such that the PMD of the transmission line is compensated by the emulator.To this end the emulator must be designed so as to reproduce the DGD of the transmission line as accurately as possible. Moreover, the emulator must emulate a “compensation line” that exhibits an “inverse” PMD behavior. For this to happen, the fast output PSP of the transmission line must coincide with the input PSP of the emulator (of the compensation line) and vice versa.        
Consequently, for a first-order PMD compensator it is important to                generate an adjustable DGD, and        rotate the PSPs of this adjustable DGD in all possible directions.        
A second-order PMD compensator should also be able to emulate a wavelength-dependent DGD with wavelength-dependent PSPs, the PSPs again having to be freely rotatable.
A main component of a PMD compensator is thus the PMD emulator which, also in a stand-alone position, can be employed in a statistically significant way as a multiply adjustable, low-cost and low-loss component for emulating the frequency-dependent polarization transmission behavior of optical fiber lines of several 1,000 km in length at different points in time and different temperatures. Known PMD compensators are either incomplete because the type of the targeted control is not clarified, have a high level of optical and electronic components, or do not function satisfactorily. Products ready for the market are, to date, not known anywhere in the world.
A reason for this is on the one hand that in the past there was no measurement device available for PMD-related distortions designed sufficiently fast and sufficiently simple. Also, no emulator is known which can reproduce—with an acceptable adjustment expense—exactly the PMD of a real transmission line.
Typical requirements to be met by a PMD compensator or PMD emulator for optical transmission lines are:                a large compensatable range of, for example, 0 to 100 ps,        ability to correct down to a residual PMD that is as low as possible,        quick correction in case of fluctuations on the transmission line,        a safe control behavior for any type of PMD,        no remaining of the control in local minimums        a low insertion attenuation, and        a low variance of the insertion attenuation.        
For an introduction to the problems underlying the present invention, FIG. 13 shows, in a schematic representation, the influence of polarization mode dispersion (PMD) on an optical transmission line. The transmission line may be a long optical fiber line from a single-mode fiber with optical signal amplifiers installed in between, as are typically used for high-rate signal transmission systems, such as OC-192 or OC-768. As described above, an SMF exhibits two mutually orthogonal polarization modes. For production, design and/or installation reasons the fiber core of the SMF is not fully isotropic with regard to its refractive index. Instead, a low direction-dependency of the refractive index, for example as a result of mechanical deformation of the SMF, is unavoidable in practice.
This creates a mode-dispersion behavior of the transmission system which is referred to as polarization mode dispersion (PMD). PMD includes all polarization-dependent transit time effects for which the signal propagation may be described fully by the propagation behavior of two polarization modes that are independent of each other and orthogonal to one another. The signal components of different polarizations of the optical signal travel through the optical fiber at different group velocities. At the receiver the light components therefore arrive time-delayed with respect to one another; this transit time effect leads to a broadening of the received signal and thus to an impairment of the transmission quality. This can particularly lead to an increase in the bit error rate.
Since the described influence on the double refraction also depends significantly on the wavelength, the position of the principal states of polarization (PSP) and the transit time difference between the PSPs also exhibits a pronounced wavelength dependency. This is also referred to as second-order PMD (SOPMD or PMD2).
FIG. 13 illustrates the influence of a PMD-affected optical transmission line on a binary data signal. The signal is coupled into the transmission system with a state of polarization SOPIN which does not coincide with an input PSP of the system. Consequently, there is a power split of the optical signal into the two orthogonal PSPs. Because of the different group velocities of the PSP1 and the PSP2 modes, there is a group transit time delay, which is referred to as differential group delay (DGD) and which represents the first-order PMD. It manifests itself in a time shift of the PSP1 mode relative to the PSP2 mode. The SOPMD additionally leads to a broadening of each individual PSP mode.
The effects mentioned result in a wavelength-dependent PMD behavior, fluctuating over time, with time constants in the ms range up to the minute range. Such PMD-related distortions in optical transmission systems must be compensated for in high-rate data transmission in order to retain the signal quality. For this purpose—but also in a stand-alone position—PMD emulators and PMD measurement devices are required.
The PMD of a transmission system can, in simplified terms, be described by the square mean value of its wavelength-dependent DGD. Equally wavelength-dependent are the input PSPs and the output PSPs of the fiber. The situation relevant to an emulation device may be described in a first approximation by the DGD and the output PSP at the mean wavelength λ0 of the optical signal.
What is desired is an emulation device for emulating a polarization mode dispersion PMD of an optical transmission system which can reproduce as accurately as possible the polarization transmission behavior of an optical transmission line with the adjustability being simple.